Kanon (2006) Episode 20: The Nocturn of Farewell
Episode 20 of 24 of Kanon (2006), aired February 15, 2007, 24 minutes. Available with English subtitles and dub where released.
Episode summary
As Yuichi and Ayu grow closer, Nayuki feels that she is being neglected by him. Ayu finally leads Yuichi to where her "school" is in the woods, but all they find is a large clearing with a tree stump in the center. The realization is too much for Ayu, and the unexpected happens right before Yuichi — she disappears.